Non-inductive filament configuration



2,1969 5. F. VOGEL 3,461,338

NON-INDUCTIVE FILAMENT CONFIGURATION Filed Jan. 16, 1967 [Ill/HUME. SIEGFRIE D FRIEDRICH VOGEL ATTORNEY United States Patent 015cc 3,461,338 Patented Aug. 12, 1969 US. Cl. 313-309 3 Claims ABSTRACT OF THE DISCLOSURE A point source filament configuration having a plurality of wire elements leading to the emission tip to provide multiple current paths for dividing the filament heating current in a manner to cancel the resulting magnetic fields of each in the area adjacent said emission tip, and thereby provide a filament configuration which is non-inductive in this area.

Cross-references to related application This invention relates to electron beam devices, and particularly relates to such electron beam devices as those disclosed in the US. patent application Ser. No. 575,731 entitled, Electron Beam Column and filed on Aug. 29, 1966, and the US. patent application Ser. No. 575,730 entitled, Automatic Electron Beam Focusing System and filed on Aug. 29, 1966. Both of these applications are assigned to the same assignee as this application.

Background of the invention In devices wherein particles, and especially electrons, are emitted from a heated filament, it is necessary to form the emitted electrons into a single beam and direct them along an axi toward a target. For precise control of the electron beam, it is necessary also that the beam be held in close alignment with a predetermined axis of the device. However, it has been found that the magnetic field created by the current passing through the filament is sufiicient to substantially divert the electron beam in the vicinity of the filament making alignment of the beam more difiicult.

Naturally, where a direct current is passed through the filament, the diversion of the beam is substantially constant with each filament. However, with the changing of the filaments, the angle of diversion usually is changed since the precise mechanical configuration of each filament is not the same. In the instances where an alternating current is passed though the filament, the magnetic field created by the filament current reverses in polarity as the direction of filament current reverses thereby causing the electron beam to be diverted first in one and then in the opposite direction. Naturally, this creates even more of an alignment problem within the electron beam device by requiring that the alignment mechanism function to compensate for beam misalignment in more than one direction from the axis of the beam device. With AC heating of the filament, the beam suffers a directional modulation. Thus, portions of the beam are cut off by apertures positioned along the beam axis thereby resulting in an undesirable current modulation of the beam.

In the past, efforts have been made to provide noninductive wire configurations such as that disclosed in US. Patent 3,259,784 filed Dec. 23, 1963 and entitled, Non-Inductive Wire Configurations. However, in electron beam devices, a point source is essential to form properly an electron beam. If a point source is not provided, then usually only a small segment of the electrons emitted can be formed into a beam with the remaining electrons being blanked off, or in the alternative elaborate mechanisms such as a number of successive lenses must be provided for directing the flood of electrons into a beam of small cross-sections. Neither of these answers has been found suitable for use in most electron beam devices, thereby making the use of a point source filament necessary. Also in electron microscopy, the use of a point source is not only desirable for its high brightness, but also for the larger degree of coherence of the beam. Thus, the resolution of device is increased considerably by the point source filament.

Summary of the invention This invention provides a non-inductive point-source filament configuration wherein a pluarity of wire elements are provided for transmitting current to the emission tip of the filamet, in combination with means for dividing the current flow such that the magnetic field created by the current passing through each of the elements cancels the fields of the other elements near the emission tip to provide a filament configuration having substantially no magnetic field which can divert the particle flow emitted therefrom. The object of this invention is to provide a point-source particle emitting filament free from magnetic fields which can divert the particles as they are emitted therefrom.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

Brief description of the drawings FIGURE 1 shows one type of prior art point-source filament.

FIGURE 2 shows one preferred embodiment of the subject invention with the circuit diagram being shown in schematic form.

FIGURE 2a is a side elevation view of the embodiment of FIGURE 2.

FIGURE 3 is a perspective view of a second embodi ment of the invention with the current diagram being shown in schematic form.

FIGURE 3a shows a top plan view of the embodiment of FIGURE 3.

Description of the preferred embodiments =In FIGURE 1 is shown a prior art embodiment of a point source filament comprising a filament 10 suitable for emitting a beam of electrons from an emission tip 11. The tip 11 is supported on a pair of wire elements 12 and 13 which are formed by the bending of one wire at a juncture 14 to which the tip -11 is attached. Such a filament is caused to emit electrons by passing filament heating current I through the segments. The arrows 15 show the direction of the magnetic fields created by the DC filament current I indicated by the arrows 16 passing through the elements 12 and 13 of the filament. It will be noted that this magnetic field extends around the tip 11 and naturally serves to divert the flow of electrons passing from the tip away from their normal path.

-It has been shown that with a current of approximately three amperes passing through a standard V-shaped filament, the deviation of the electrons emitted from the filament can be as much as 3 from the original path taken by the electrons. Naturally, if alternating current is applied to the filament, the actual angular difference in the path taken by the electrons as the filament current is reversed is 6. Thus, substantial care must be taken to maintain the electron beam in proper alignment with the axis 'of the electron beam device employing the filament.

In accordance with the present invention, a point source particle emitting filament is provided wherein a plurality of segments are joined at one end to carry the filament current to and from the emitting tip and with means provided to divide the current flow between the segments such that the magnetic fields resulting from the flow of filament current to and from the tip are symmetrically disposed about the electron emitting tip and of opposite polarities thereby to cancel each other for providing a field-free point-source filament for emitting particles such as electrons.

A preferred embodiment of the invention is shown in FIGURE 2 wherein one element 17 of the filament is formed by a straight wire segment while a pair of elements 18 and 19 are formed by the bent wire segment. The element 17 is attached by an electrical conduction means such as welding near one end to the bend between the elements 18 and 19 forming the juncture 20, and such that it extends past the juncture to form a point source tip 17a for emitting electrons which move along a path coinciding with the axis 17b of the filament. Since this tip is immediately adjacent to the juncture where the heating current passes, the tip is heated by conduction along the tip to the point source. In the normal manner of operation, the majority of electrons emitted from a filament are emitted from the conical portion of this tip since the electrostatic field normally employed in the beam device for accelerating the electrons in a direction away from the filament along the filament axis is the strongest at this point.

Means are also provided for dividing the filament heating current in a'nianner to equalize and thereby cancel the magnetic fields resulting from the currents passing through the elements of the filament to and from the juncture 20, respectively. To accomplish this, the elements 18 and 19 are connected together by a conductor 20 which, in turn, connects through the conductor 21 to a DC voltage source 22. The other terminal of the source 22 is connected by the conductor 24 to the element 17 of the filament. Thus, current passes through the element 17 of the "filament (as indicated by the arrow 25) to reach the juncture 20 and thereafter divides to be returned to the voltage source through the elements 18 and 19 as indicated by the arrows 26 and 27.

The current passing through the filament elements heats up these elements, which heat is conducted to the tip 17a to cause electron emission therefrom. Since the juncture 20 can be designed to represent a high resistance point of the filament, most of the heating takes place at this point resulting in a high temperature zone being positioned immediately adjacent the base of the tip.

The arrows 28 and 29 indicate the magnetic fields created by the flow of electric current through the elements 18 and 19 of the filament. Since theseelectric currents are of substantially equal magnitude, the magnetic fields are also of equal magnitude. Thus, in the center area of the filament adjacent the element 17 which is positioned on the filament axis, the fields are of opposite polarities and thereby cancel each other since the elements are on opposite sides of and equidistant from the axis 17b. There still exists an additive magnetic field created by the electric current passing through both elements 18 and 19, which field is indicated by the arrow 30a. However, a field 30b of equal strength is created by the current fiow through the element 17. This field is of equal strength since it is generated by the sum of the currents passing through the elements 18 and 19. Additionally, as indicated by the arrow 30b, this field is of opposite polarity to that indicated by the arrow 30a since the current fiow creating the field is in the opposite direction to that creating the field 30a relative to the filament. Thus, the magnetic fields created 'by the filament heating current in being also symmetrically disposed about the axis 17b, cancel each other. Therefore, no magnetic field appears adjacent to the emission tip 17a which could otherwise divert the electron flow therefrom, and a magnetic free or non-inductive filament is provided.

As an added benefit to using the multiple paths for the electric current, the life of the filament is extended. This extended life is achieved since with evaporation of either the elements 18 or 19, the cross section of the wire becomes smaller which increases the evaporation process in normal filaments since the current flow therethrough remains substantially constant and the higher resistance area creates a greater heating effect. However, as the resistance of this element increases and the current therein decreases by a proportional amount because of the parallel circuit connection through the elements 18 and 19 thereby to maintain the heating of the element substantially normal. Thus, the evaporation rate is maintained substantially equal to that of the other element to yield a maximum useful life from the filament. It should be noted that this elfect is self-regulating since the division of current flow through the elements 18 and 19 always is proportional to the resistance thereof, which resistance regulates the heating (and thereby the evaporation) of each element.

A third embodiment filament 33 of the invention is shown in FIGURE 3 wherein four elements 34, 35, 36 and 37 are joined at one end to form a juncture 38 to which is fastened a point source or electron emitting tip 39in a position coinciding with the filament axis 39a. The elements 34 and 35 are connected in a parallel circuit combination to one terminal of a current source 40 by a conductor 41, while the current receiving ends of the elements 36 and 37 are connected in a parallel circuit combination to the other terminal of the current source by a conductor 42. Thus, current is passed through the elements 34 and 35 in a direction indicated by the arrows 44 while the current passing through the elements 36 and 37 flows in a return direction indicated by the arrows 45.

In FIGURES 3 and 3a, the fields created by the currents 44 in the elements 34 and 35 are indicated by the arrows 46 while the fields created by the currents 45 are indicated by the arrows 47. Since the electrical resistances, and therefore the current flow through the elements 34 and 35, and the elements 36 and 37 are substantially equal, the magnetic field-s 46 and 47 are also substantially equal. Thus, each of the fields cancels out the other within the interior area of the filament. The fields of each of the electric currents 44 and 45 are additive to the exterior of the filament to create the magnetic fields indicated by the arrows 48 and 49. However, these fields are created by the total filament current passing through the elements and are of opposite polarity because of the reversal of direction of current flow in each pair of elements. Thus, the external fields also cancel each other making the filament 33 also magnetic field-free, and thereby non-inductive. It is obvious that many other combinations of filament elements can also be devised to embody the same principle of cancelling the magnetic fields generated by the filament current How. The requisite for any such magnetic field-free filament is that the magnetic fields created by all the currents passing to and from the juncture adjacent the emission tip be of equal strength and of opposite polarity so as to cancel each other at least in the vicinity of the path taken by the emitted particles. Thus, the elements need not necessarily be symmetrically disposed about the filament axis, however, such are the prferred embodiments as shown in the drawings. Additionally, the elements need not be of equal cross-section since the size thereof is determined primarily by the magnitude of the current conducted thereby.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

I claim as my invention:

1. A filament as defined in claim 3 wherein said juncture of said elements corresponds in position with said filament axis.

2. A filament as defined in claim 3 wherein the unjoined ends of said elements extend generally in the opposite direction from the direction of the emission path taken by said emitted particles.

3. A non-inductive particle emitting filament compris mg:

a plurality of electric conductor elements,

electrical conduction means joining said elements at a common juncture with the unjoined ends of said elements extending away from said juncture,

a portion of said elements adapted to generate heat with the passage of electric current through said element,

a point-source particle-emission surface in heat transfer relationship with said elements portion formed by the end of one of said elements extending past the common juncture in a position corresponding to a filament axis extending along the path of particle emission from said filament,

and means to supply electric current to selected ones of said unjoined element ends and to receive electric current from the remaining element ends with the magnitude of the current passing through each element being regulated such that the magnetic fields created thereby all cancel each other in the area adjacent a portion of said filament axis to provide a magnetiofield-free path for said emitted particles.

References Cited UNITED STATES PATENTS Charibonnier et al. 313-336 X JOHN W. HUCKERT, Primary Examiner A. J. JAMES, Assistant Examiner US. Cl. X.R. 

